Uric acid is a metabolite of purines in animals and human. For human, uric acid is excreted as the end-product of purine metabolism through the intestine and kidney in human body due to the lack of uricases which continue to oxidatively degrade uric acid to more water-soluble allantoin, and renal excretion is the main pathway for uric acid excretion in human body. The upper limit of normal uric acid concentration range in the human body is 400 umol/L (6.8 mg/dL) for male and 360 umol/L (6 mg/dL) for female. Aberrant levels of uric acid in the human body are often due to the increased production of uric acid or decreased excretion of uric acid, which usually include three types: increased uric acid production type, reduced uric acid excretion type and mixed type. Disorders closely related to aberrant levels of uric acid include hyperuricemia, gouty arthritis (also referred to as gout), kidney stones, urinary calculi, hypertension, etc.
Hyperuricemia refers to a disease in which the metabolism of purine substances in the human body is disordered, resulting in an increased uric acid production or a decrease in excretion, and an aberrantly high level of uric acid in the blood. When the concentration of uric acid is more than 7 mg/dL in human blood, uric acid is deposited as a monosodium salt in the joints, cartilage and kidneys, resulting in overreaction (sensitivity) of the body's immune system and causing pain, this symptom is called gouty arthritis. The general attack sites of acute gout are peripheral joints such as the big toe joint, ankle joint, knee joint and so on, and red, swollen, hot, and severe pain appear in the attack site of acute gout, which usually occurs in midnight and can make people wake up from sleep. Hyperuricemia is the pathological basis of gouty arthritis, and the use of drugs to decrease blood uric acid concentration is one of the common methods for preventing gouty arthritis.
In recent years, the attack of hyperuricemia and gout disease is on the rise as the change of lifestyle. In Europe and USA, researches on the epidemiology have shown that the incidence of gouty arthritis accounts for 1-2% of the total population and is the main type of arthritis in adult males. Bloomberg News estimates that there will be 17.7 million gout patients in 2021. In China, the survey shows that 25.3% of the population has a high blood uric acid concentration and 0.36% has gout diseases among the population aged 20 to 74. At present, clinical treatment drugs mainly include 1) inhibition of uric acid-producing drugs, such as xanthine oxidase inhibitors allopurinol and febuxostat; 2) uric acid excretion drugs, such as probenecid and benzbromarone; 3) inflammation inhibitors, such as colchicine and so on. These drugs have certain defects in treatment, including poor efficacy, large side effects, and high cost are some main bottlenecks in clinical application. It has been reported that blood uric acid levels of 40%-70% of patients who have received standard treatment did not meet the expected therapeutic goals (<6 mg/dL).
URAT1 is an important renal anion transporter located on the brush border membrane of the epithelial cells of the renal tubules, specifically transporting uric acid from the renal tubules to epithelial cells, which is the main driving force for uric acid reabsorption in the renal tubules. Therefore, if the urate transporter URAT1 can be significantly inhibited, it will increase the excretion of uric acid in the body, thereby lowering blood uric acid level and reducing the possibility of gout attack.
The first URAT1 target inhibitor Leinurad of AstraZeneca showed in the figure below was approved by the FDA in December 2015. The 200 mg/day dose was approved in combination with xanthine oxidase inhibitor XOI (such as Febuxostat, etc.) for the treatment of hyperuricemia and gouty arthritis, but the additive effect of combination was not very significant compared with the xanthine oxidase inhibitor alone. The 400 mg/day dose of Leinurad was not approved due to significant toxic side effects at high doses (the incidence of kidney-related adverse events, especially the incidence of kidney stones), although the higher additive effect of combination appeared. Therefore, the FDA required the Leinurad label to be filled with a black box warning to warn the medical staff acute kidney failure caused by Leinurad, especially when not used in combination with XOI, and if the over-approved dose of Leinurad was used, the risk of renal failure is even higher. Meanwhile, the FDA asked AstraZeneca to continue its evaluation on kidney and cardiovascular safety after Leinurad marketed. For long-term drug use for a metabolic disease, the safety of the drug is particularly important. Therefore, there is a strong demand to develop a safe drug for lowing blood uric acid.

In the new drug declaration report disclosed by AstraZeneca, the results of the identification experiments of compound Lesinurad in liver microsomes and hepatocyte metabolites of various animal species in vitro were reported in detail. The data showed that M3 and M4, two major metabolites of Lesinurad was significantly detected in the monkey and human hepatocytes, but M3 and M4 were not detected in dog and rat hepatocytes, as shown in Table-1 below.
TABLE 1SystemSpeciesM3M4LesinuradTotalliverrat——100100microsomedog——100100monkey7.9 —92.1100human——100100hepatocyterat——100100dog——100100monkey1.450.4798.1100human2.245.6992.1100
Meanwhile, AstraZeneca also reported the main metabolites and metabolic pathways of Lesinurad after administration in various genus animals, in which the bishydroxy metabolite M4 was specifically detected in human metabolites:

This was consistent with Lesinurad's clinical data. Experimental data showed that M3, M4 were the main metabolites found in human clinical, as shown in Table-2 below.
TABLE 2TimePercentage of administered doseSystem(h)M1M2M3M3bM4M5M5bM16OtherLesinuradTotalUrine0-1441.50.312.01.015.7NDND0.51.231.363.4Feces0-144ND4.80.31.95.03.67.81.17.51.533.5
The production pathway of M4 metabolite could be determined as a result of the co-action of cytochrome CYP2C9 and primate epoxide hydrolase mEH. This mEH metabolic pathway was unique to primate species, which explained why no M4 was observed in rats and dogs:
